This invention relates to the catalytic conversion of hydrocarbons, particularly high boiling hydrocarbon oils such as gas oils, to hydrocarbons of lower boiling range.
Hydrocarbon conversion processes utilizing crystalline zeolites and, in particular, crystalline aluminosilicate catalysts, have been the subject of considerable investigation as is apparent from both the extensive patent and scientific literature. Large pore crystalline silicate zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes including the catalytic cracking of a gas oil to produce motor fuels and have been described and claimed in many patents, including U.S. Pat. Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. As recognized in these patents, the as-synthesized zeolites, i.e., the zeolites substantially in the alkali metal form, are not suitable for cracking. The selectivity of alkali metal zeolites is extremely poor compared to the older amorphous silica-alumina cracking catalysts. In addition, certain alkali metal zeolites are quite unstable to steam treatment, such treatment being a necessary consequence of undergoing regeneration in a catalyst regeneration zone. For these reasons, the natural and synthetic alkali metal zeolites as found or synthesized must be modified in order to be rendered suitable for use in cracking operations. Specifically, the alkali metal zeolite must be base-exchanged to replace a substantial part of the alkali metal with protons. Alternatively, other cations, e.g., rare earth metal cations, can be used to displace alkali metal as disclosed in aforementioned U.S. Pat. No. 3,140,249.
U.S. Pat. No. 3,530,064 discloses a catalytic cracking process employing a large pore zeolite of relatively low alpha value such as dealuminized zeolite Y and a selective combustion catalyst such as platinum zeolite T. At least part of the heat demand of the endothermic cracking reaction is supplied in situ by the selective pyrolysis of a portion of the feed and/or reaction product. There is no recognition or appreciation in this patent that relatively minor amounts of oxygen will promote cracking with alkali metal zeolites.